An elastic neutron scattering study has been performed on several single crystals of La$_{ 2-x}$Sr$_{ x}$CuO$_{ 4}$ for { \it x} near the lower critical concentration { \it x$_{ c}$} for superconductivity. % In the insulating spin-glass phase ({ \it x} = 0.04 and 0.053), the previously reported one-dimensional spin modulation along the orthorhombic { \it b}-axis is confirmed. % Just inside the superconducting phase ({ \it x} = 0.06), however, two pairs of incommensurate magnetic peaks are additionally observed corresponding to the spin modulation parallel to the tetragonal axes. % These two types of spin modulations with similar incommensurabilities coexist near the boundary. % The peak-width $\kappa$ along spin-modulation direction exhibits an anomalous maximum in the superconducting phase near { \it x$_{ c}$}, where the incommensurability $\delta$ monotonically increases upon doping across the phase boundary. % These results are discussed in connection with the doping-induced superconducting phase transition.
Categories: cond-mat.supr-con cond-mat.str-el